Appearance of lines due to $K \neq 0$ oscillations in lattice vibrations spectra of paradibrombenzene nanoparticles

Abstract

We measured low-frequency Raman spectra of paradibromobenzene nanoparticles. With the reduction of the nanoparticles size frequencies of lines become smaller and the doubling of the most intensive lines takes place. Additional lines with increasing intensity appear in the spectrum. We did structure calculations for nanoparticles by the method of molecular dynamics and obtained histograms of lattice vibrations spectra by the Dyne's method. Calculations have shown that doubling of lines is due to the appearance of nanoparticles vibrations from borders of the Brillouin zone in the spectrum. Additional lines are caused by vibrations with $K \neq 0$. They have mixed orientation-translational character (that is evident from the eigenvectors of vibrations) that affects intensity of these lines. Upon reduction of the particles sizes change of vibration eigenvectors of molecules occurs $K \neq 0$. The greatest changes observed for a 20 cm$^{-1}$ paradibromobenzene line.